The present invention relates to a process for the preparation of a heat-sensitive recording material, particularly a diazo-based recording material. More particularly, it is concerned with a process for preparing a diazo-based recording material which causes less formation of background fog.
As a heat-sensitive recording material, a leuco color formation-type heat-sensitive recording material is usually used. Such a recording material, however, has a disadvantage in that rough handling, heating, or contact with solvents after recording causes color formation in unexpected areas, thereby making a recorded image dirty.
In order to develop a heat-sensitive recording material which is free from the above problems, extensive investigations have been made on a diazo color formation-type heat-sensitive recording material in recent years. For example, a heat-sensitive material containing a diazo compound, a coupling component and a basic component (including a precursor substance which becomes basic upon application of heat) in which after heat recording, irradiation with light is applied to decompose an unreacted diazo compound, thereby stopping color formation is disclosed in Japanese Patent Application (OPI) No. 123086/82 (the term "OPI" as used herein means a "published unexamined Japanese patent application"), Gazo Densi Gakkaishi (Journal of Image Electronics Association), 11, 290 (1982), and so forth. In accordance with such method, the formation of color in areas where recording is not needed can be stopped (this stopping is hereinafter called "fixation"). In this recording material, however, pre-coupling occurs, causing undesirable color formation, i.e., fog. In order to prevent this pre-coupling, an attempt to prevent contact between components by introducing any one of the color-forming components in the form of non-continuous particles, i.e., solid dispersion has been made. This attempt, however, meets with disadvantages in that not only can the formation of fog not be completely prevented but also heat color forming properties are reduced. As another attempt, a method of minimizing the contact between components by incorporating the diazo compound and the coupling component in different layers is known (see, for example, Japanese Patent Application (OPI) No. 123086/82). This method, however, is not suitable for practical use because although the fog is somewhat reduced, heat color-forming properties are seriously reduced and the recording material cannot respond to high-speed recording having a short pulse width.
As a method permitting the prevention of fog while maintaining heat color forming properties, it is known to encapsulate either the coupling component or the basic substance with a non-polar waxy substance (Japanese Patent Application (OPI) Nos. 44141/82 and 142636/82) or a hydrophobic polymeric substance (Japanese Patent Application (OPI) No. 192944/82) to thereby isolate them from each other. In this encapsulation method, however, the wax or polymeric substance is dissolved in a solvent and then a color-forming component is dissolved or dispersed in the resulting solution to thereby form capsules. These capsules, therefore, are different from those of the usual capsule type. For this reason, when the color-forming component is dissolved, it does not constitute the core of capsules; rather, it uniformly mixes with the encapsulation substance and undergoes pre-coupling in the interface between the capsules, thereby forming fog. When the color-forming component is dispersed, the color-forming reaction does not occur unless the capsule walls are melted by heating, and thus heat color-forming properties are reduced. Furthermore, after formation of capsules, it is necessary to remove the solvent used to dissolve the wax or polymeric substance. This requires an additional step in preparation of capsules and therefore the above method is not sufficiently satisfactory.
In order to overcome the above problems, Japanese Patent Application (OPI) No. 190886/84 discloses a heat-sensitive recording material in which at least one color-forming components is used as the core substance and walls are formed around the core substance by polymerization to achieve microencapsulation.
With conventional capsules, the capsule walls are broken by applying heat or pressure to thereby bring a reactive substance contained in the core of the capsule into contact with another reactive substance outside the capsule, while with the capsules prepared by the above method, the reactive substances present in the core of the capsule and outside the capsule are reacted mainly by allowing them to permeate through the capsule walls by heating.
In this way, color formation can be accomplished by the coupling reaction between the diazo compound and the coupling component according to the above procedure in which the diazo compound is incorporated in the microcapsule and the capsule walls are made to be permeable to the components when heated.
However, in preparing microcapsules containing the diazo compound, some of the diazo compound remains unincluded or unincorporated in the microcapsules and exists in the solution, and this unincluded diazo compound sometimes causes background fog even after preparation of a recording material. To overcome this problem. Japanese Patent Application (OPI) No. 214992/85 laid open on Oct. 28, 1985 corresponding to U.S. patent application Ser. No. 06/721,521 filed Apr. 9, 1985 discloses a method of adding a coupling reaction-deactivating agent to a solution containing the microcapsules. This method, however, is not sufficiently satisfactory.